The coupling of surface membrane depolarization of heart cells to contraction is mediated by a release of Ca2+ from the sarcoplasmic reticulum (SR). The precise mechanism of coupling of the depolarization to SR Ca2+ release is presently the preeminent issue in cardiac biophysics. Morphological studies of cardiac cells have shown that the junctional SR, from which Ca2+ is released upon stimulation, forms structural "couplings" with the sarcolemma (SL), primarily (by 80%) with the T tubules, ie. invaginations of the SL into the cell interior. At the "couplings", the SL Ca2+ channels and the SR Ca2+ release channels span the gap between closely apposed but separate SL and SR membrane systems. It has been generally postulated, therefore, that a direct regulatory effect of the L-type Ca2+ current (ICa ) on the SR Ca2+ release, typical for mammalian ventricular myocytes, results from a preferential access of the Ca2+ current to the SR Ca2+ release sites via the "couplings". The present project is aimed on testing this hypothesis by comparing the dependence of SR Ca2+ release on the L-type Ca2+ current in mammalian (rat) ventricular myocytes, which have an extensive T tubular system and extensive "couplings", and avian (Finch) ventricular myocytes, which lack the T tubules and, therefore, over 80% of its junctional SR lacks contact with the SL. Rat and Finch ventricular cells are isolated from the heart (collagenase digestion), loaded with a Ca2+ indicator (indo-1/FA) via a patch pipette, and voltage clamped (whole cell patch method). Test depolarizations lasting 25 to 250 msec to potentials that span the range of the L-type Ca2+ channel activation are then made and ICa and Ca2+ transient amplitude are measured. The results obtained so far show that in bird ventricular myocytes the SR Ca2+ release is well correlated to Ca2+ influx via the L-type Ca2+ channels and is qualitatively similar to the relationship between ICa and the SR Ca2+ release observed in rat ventricular myocytes. Thus, the results suggest that the direct control of SR Ca2+ release by ICa is not attributable to a preferential access of Ca2+ influx to SR Ca2+ release sites via the SL-SR "couplings". This apparently necessitates a change of the current dogma regarding the functional significance of these "couplings". Specifically, models of excitation-contraction coupling in which Ca2+ diffusion is more extensive perhaps in series with local control models (20% of coupling in Finch) will need to be explored.